Problems with Many Worlds Interpretation

[Fyzix]

I think decoherence-based interpretations are the superior to collapse-based interpretations for understanding quantum mechanics. Among those, MWI appears to be the most promising approach for understanding the quantum-classical interface -- but that purpose does remain conjectural until quantum thermodynamics is sufficiently well-developed.

Also, I suspect that the MWI has more ways to fail than the CI. Since it hasn't failed, that would imply MWI enjoys a greater share of empirical support than CI.

Insomuch as one uses science to understand reality, MWI seems to be the understanding that QM tells us.

But what about the preferred basis problem ?
Also sure you thought of something similar as Tegmark when it came to Born Rule, but it still requires infinity.
Last but not least, ontology objection from Maudlin?

 

[Ken G]

To the best of my knowledge, "collapse-based interpretation" and "decoherence-based interpretation" are standard phrases in the taxonomy of interpretations. I'm 99% sure I didn't invent the terms.

It was never my claim that you invented the misnomer, I merely gave an argument, which you did not address, why it was a misnomer. If you hold that it is not a misnomer, please explain CI to me without decoherence. The irony is, people who criticize CI invariably describe it in a way that leaves out decoherence, and then they blame CI for not being informed by decoherence. The actual fact is, CI needs decoherence just as badly as MWI does, as anyone who understands CI knows. If there were times when Bohr didn't get decoherence, he would have been troubled by something missing from his perspective, and then when he did get decoherence, he would have said "ah, exactly, now what I've been saying all along finally becomes clear." Decoherence is what enables a subsystem in a pure state to become a mixed state when the closed system is projected onto that open subsystem, all prior to the perception of that subsystem. CI is how we interpret the resulting mixed state, prior to said perception (a perception that will return the substate to a pure state). Now, please tell me what is "not decoherence based" about that?

But the distinguishing point is that you don't interpret the resulting mixed state as referring to a parmstate, but instead interpret it as expressing ignorance.

Precisely. But we must go a step farther-- we must understand what "ignorance" actually is. To understand ignorance, you must have a model for knowledge. That means you have to choose empiricism or rationalism. CI chooses empiricism. This is the crucial element, because if one asserts that observation is truth, then one must frame knowledge of truth in terms of what is observable. That means one is not building a rationalistic model of "ignorance" (which doesn't work very well from the rationalist god's eye view where ignorance is an odd concept), one is saying that ignorance means not knowing how an observation will come out. But not knowing that means there is something we don't know about perception, it all comes down to perception because that is what an empiricist means when they talk about an "outcome" of an observation. Empiricism requires that we include the act of perception in our ontological description of reality, rationalism does not require that. This is the crux of the distinction between MWI and CI, it is absolutely inescapably intertwined with the different knowledge models of rationalists and empiricists.

I feel like people are effectively saying:

I don't like quantum mechanical view, so I will try my best to make do with pre-quantum views until the next scientific theory comes along, which I tacitly hope with be more palatable.​

No, you are quite wrong here. Really couldn't be much wronger, to be honest. You think that empiricists don't like quantum mechanics. How could you possibly think that Bohr and Heisenberg didn't find quantum mechanics palatable? They loved quantum mechanics, as do I. I'm mostly an empiricist, and I think quantum mechanics is just splendid, fascinating, amazing. I also think it is probably not exactly right, and as I said, I think unitarity is probably only nearly unbroken. None of that has anything to do with "not liking" QM, it has to do with a respect for the history of science, and a basic vein of skepticism that runs through me. I agree that empiricists are generally more skeptical of theories, whereas rationalists tend to marry them after one date, but that has nothing to do with how much they like them-- it has to do with what they think truth is.

You think empiricists don't like QM, and that's why they take a skeptical stance toward it, but your mistake about that motivation is actually revealing your own motivation for not liking empiricism! Empiricists would have adopted skeptical stances around every theory, including classical mechanics, so it has nothing to do with liking classical thinking better. Instead, it has to do with a basic recognition that physics is what we can say about nature, so we are in the conversation, including the fact that we are "classical" systems (which does not mean we are systems that obey classical laws, because empiricists don't think we really obey laws at all, they think laws are how we understand and predict behavior). If there aren't laws that "govern" nature, then the act of seeking laws is something quite a bit different from how rationalists frame that process, and this has nothing to do with the details of any theory.

For the record, I think "QM-style theories keep working all the way up", "new physics that produces collapse and works all the way up" and "new physics diverges even further from the classical intuition but works all the way" are all perfectly good outcomes.

Yes, at the end of the day we all just want the truth. Still, my point is that it is always questionable to build a world view on any particular set of postulates, it is just taking those postulates too seriously and it has never been right yet. It might be a valid constraint to impose when seeking the next theory, I view the seeking of theories to be the valid application of rationalism. It is the building of world views that I classify as rationalism taken to a naive extreme.

As an aside, doesn't this position essentially boil down to "I think that QM is explained by a hidden-variable theory?"

But you are again coming from a rationalistic perspective! You think that for QM to be wrong, some other theory has to be right. The empiricist just doesn't think that way-- there is no right theory, physics is about what we can say about nature, and that is never going to be nature herself. We get observations to tell us what is true, and we get theory to help us predict what will happen, and we get a limited sense that we understand why it happened. That's it, that's science-- that's all we get.

And even if it turns out that a quantum mechanical world view would need to be discarded 25 years down the road, so what? You're almost surely better off having used a quantum mechanical world view for those 25 years than you would have been struggling along with a pre-quantum world view.

Again, you see the options as "quantum world view" or "pre-quantum world view." Those are just the two possibilities open to rationalists. Empiricists prefer a different approach: skepticism of all world views. They are all seen as valuable for what they are good at, yet limited in truthfulness, and certainly not to be taken seriously. When Newton's laws made the future seem to be nothing but an echo of the past, the empiricist asked "what observation proves that?" None. When MWI asserts that the universe as a whole does not change at all, the empiricist asks "what observation proves that?" None. So it goes with world views.

Remember, Einstein said "make the theory as simple as possible -- but no simpler!" Rejecting parmstates makes things too simple, because parmstates are needed to fully evaluate how QM connects to observation.

So? QM invokes wave functions, we know that, it is part of the theory. We are talking about interpreting the parmstates, not "rejecting" them from the theory altogether.

 

[Hurkyl]

But what about the preferred basis problem ?

There is no problem with the idea that talking about "splitting" relative to a basis depends on your choice of basis.

Or are you referring to the question of whether QM predicts rulers can exist? That question has to wait for quantum thermodynamics to be developed.

But I'm aware of no obstacle posed by the observed existence of rulers, nor by the operational definition that rulers are things that measure position.

Also sure you thought of something similar as Tegmark when it came to Born Rule, but it still requires infinity.

Of course it does.

Frequency probability is the interpretation of probability that defines an event's probability as the limit of its relative frequency in a large number of trials. (ref: wikipedia)​

Last but not least, ontology objection from Maudlin?

I don't even know what this is.

 

[Ken G]

Wonderful. I just flipped a coin, and my observation was parm -- I saw heads.

You saw heads, that's all you get. What do you mean it was parm? I don't have to use observations to prove to you what you didn't observe, I just want to know what you did observe. Did you observe that heads was parm? How did you observe that?

 

[Ken G]

Alien: "Oh, that's right. I heard you have some weird colors... blue and green, I think. I heard your blue color is normally our bleen, but at until noon today, your blue color switches to grue. And conversely for green. Where did you ever get such a strange notion of color anyways? Weird coincidence that your colors switch the same time the sky and ground swap colors.

This presents no paradox to the empiricist, actually. Observations are of consistencies, not properties. We don't need a rationalist description of what green "really is" to say that we perceived it, the consistency of the perception provides sufficient basis for counting it as part of what is true. Yes, this means perception remains fundamentally a mystery, and yes, it means perception is something our brains are doing not something "out there", but that's just how it is. We won't really understand the connections between the empiricist and rationalist perspectives until we understand better what perception is, and I think its role in "collapse" is going to be the crucial avenue for reaching that understanding. Ergo, rejecting collapse merely makes it impossible to ever understand what perception is, which isn't surprising because MWI sees no significance in perception anyway.

 

[Hurkyl]

You saw heads, that's all you get. What do you mean it was parm? I don't have to use observations to prove to you what you didn't observe, I just want to know what you did observe. Did you observe that heads was parm? How did you observe that?

I saw heads.

It was parm because experiences are parm. You keep telling me they aren't parm. What do you mean? Did you observe that heads wasn't parm? How did you observe that?
This last paragraph is devil's advocate, because I know full well that (classically, at least) parmness is not observable, because the observations would share any parmness present, rather than transcending it. And, while I'm not quite sure, I think you know that too.

My observations do not include the adjective "not parm". When you tell me my observations are not parm, you are making theoretical assertions about what is true -- the very thing you are trying to criticize MWI of doing. But I don't see why MWI is more deserving of such criticism than CI.

 

[Ken G]

I saw heads.

It was parm because experiences are parm. You keep telling me they aren't parm.

No, I'm not telling you what experiences are, no empiricist can tell you that. Experiences are the ultimate mystery in empiricism. They define truth, that's it, that is the axiom. They do not have attributes like "parm" or "not parm", the words "parm" or "not parm" apply to rationalistic conceptualizations of what is going on, the experience just is. I saw heads, heads is what happened. Saying the experience is parm or not parm is a category error in empiricism.

MWI cannot stop there. It must try to say why heads happened, or heads isn't what happened, because to a rationalist, what happens is not what is experienced, but rather what is conceptually true, mathematically true. So the postulates dictate truth, and the experiences check the postulates, but have no other connection to the truth. The postulates are where "parm" comes from, not the experiences, but the experiences are consistent with the rationalist description of the parm state. I never said observations ruled out parmstates, I said that if the observations are regarded as what is fundamentally true, then parmstates are not what is fundamentally true (which is quite a bit different from saying they are false, because only observations can say what is false also). In empiricism, parmstates are neither true nor false, they are aspects of a theory. But since the parmstates are not regarded as true (or false), they cannot be regarded as real. They can be regarded as unreal on the basis of not being real, it does not require they be falsified.

This last paragraph is devil's advocate, because I know full well that (classically, at least) parmness is not observable, because the observations would share any parmness present, rather than transcending it. And, while I'm not quite sure, I think you know that too.

Yes, absolutely, parmness is not observable. One can neither observe its presence, nor its absence. That's why the rationalist view cannot be ruled out by the observations, but it is ruled out by the empiricist anyway, based on the axioms of empiricism-- that which cannot be observed is angels on a pin, and the elements of any theory are the map not the territory. Whether or not one regards the observations as the territory or just another kind of map depends on how much of a realist one is, but the fact remains, the theories change but the observed outcomes don't.

My observations do not include the adjective "not parm". When you tell me my observations are not parm, you are making theoretical assertions about what is true -- the very thing you are trying to criticize MWI of doing.

Now you are saying that empiricism does not have axioms. That is untrue, empiricism does have axioms.

 

[Hurkyl]

In empiricism, parmstates are neither true nor false, they are aspects of a theory. But not being true means they are not real.

Then where do you get off claiming that empiricism rejects MWI because it connects the theory to perception through parmstates? And how do you not manage to reject CI for the same reason -- that it connects theory to perception through collapse into non-parmness?

 

[Hurkyl]

We don't need a rationalist description of what green "really is" to say that we perceived it,

The paradox relates to why we perceive green instead of grue, why we think our experiences lead us to believe the sky is blue and not bleen.

and I think its role in "collapse" is going to be the crucial avenue for reaching that understanding. Ergo, rejecting collapse merely makes it impossible to ever understand what perception is, which isn't surprising because MWI sees no significance in perception anyway.

I am interested in understanding the appearance of collapse -- I see no reason to insist on anything stronger.

Failing to insist on something stronger does not equate to "see[ing] no significance in perception".

 

[Ken G]

Then where do you get off claiming that empiricism rejects MWI because it connects the theory to perception through parmstates?

That isn't the problem that empiricists have with MWI. The problem is treating MWI like it was real. Which MWI enthusiasts always do, by the way (witness "quantum suicide"). Empiricists who understand QM have no issue with treating MWI like a valid pedagogy for picturing how QM works, they object to taking that valid pedagogy of a particular theory and following the faulty logic "the theory agrees with observations, so not only should the theory be regarded as true in reality (yuck), but a highly rationalistic interpretation of the theory should also be regarded as true in reality (double yuck)." Not by empiricist axioms it shouldn't.

And how do you not manage to reject CI for the same reason -- that it connects theory to perception through collapse into non-parmness?

Empiricism has no problem at all with "connecting theory to perception", indeed it views that connection as the whole point of a theory. The issue is with associating the theory with what is either true, or real. In empiricism, nothing but observations are ever true or ever real, and the history of science is rather clear on the advantage of thinking that way.

 

[Ken G]

The paradox relates to why we perceive green instead of grue, why we think our experiences lead us to believe the sky is blue and not bleen.

Again, that is no paradox to empiricism, because it is not the label of the perception that matters, but the consistency of the perception. If we came out tomorrow and said "henceforth green will be called grue", no empiricist would bat an eyelash, because the consistency would be perfectly maintained under the transformation green-->grue. That is also why there is no need to establish that two observers perceive the same thing when they perceive green-- the truth of the perception of green is entirely in the consistency of the perception, which allows the observers to agree on a "green" perception. There doesn't even have to be any such thing as the "actual" perception of green, merely the consistency class.

I am interested in understanding the appearance of collapse -- I see no reason to insist on anything stronger.

Yes, empiricist are also interested in understanding that, they just drop the word "appearance" because there is no such thing as a difference between collapse and a consistent appearance of collapse. Consistent appearance = truth.

Now, to stave off any complaints about "mirages" or some such, we can well agree that a mirage is a consistent appearance. Hence it is true-- it is true that the eye is really seeing something that the brain can erroneously interpret as water, and the reason the interpretation of the observation is erroneous is because the consistency of that perception breaks down when you go and stick your toes into the mirage. This is generally not any kind of problem for physics experiments, though we certainly try to get as many consistency checks as possible, and who knows-- tomorrow we might find a consistency breakdown that suggests "the Matrix" was right all along.

Failing to insist on something stronger does not equate to "see[ing] no significance in perception".

Explain what perception is from an MWI standpoint. Does it have any importance in the theory of quantum mechanics? (Not in testing the theory, in the theory itself-- in the description of what is happening, according to quantum mechanics.)

 

[Fra]

I interpret Hurky's meaning of "apperance" so as to acknowledge the de facto collapse but denial of it at deeper level.

In terms of information update, this is the same as to acknowledge that there sort of is an information update, but that there always is a different (larger) perspective in which the information is more complete and the information update is just understood as apparent when you truncate the picture.

The problem with this picture is that "removing the collapse" requires changing perspective. And this choice is non-physical for a given observer, because it's stuck with the perspective it has.

MWIers always refer back to the master view in which there is no collapse, and from this view surely you can say that the collapse is apparent.

The problem is IMO that this master view does seem to map onto any physical views. It's more some kind of mathematical view whose value is unclear beyond maintainin a unitary picture, even though not one corresponding to a real observer.

But this seems to pretty much what the core of entire thread has been about.

/Fredrik

 

[Ken G]

In terms of information update, this is the same as to acknowledge that there sort of is an information update, but that there always is a different (larger) perspective in which the information is more complete and the information update is just understood as apparent when you truncate the picture.

That's how I'm getting him too, if he means something else he will need to clarify.

The problem with this picture is that "removing the collapse" requires changing perspective. And this choice is non-physical for a given observer, because it's stuck with the perspective it has.

That's just how I see the problem too.

MWIers always refer back to the master view in which there is no collapse, and from this view surely you can say that the collapse is apparent.

The problem is IMO that this master view does seem to map onto any physical views. It's more some kind of mathematical view whose value is unclear beyond maintainin a unitary picture, even though not one corresponding to a real observer.

I would equate that "master view" with the "rationalist view" about what constitutes knowledge of truth. The debate we are having right now dates 2500 years back to Parmenides, who was the ultimate rationalist for the ages-- he said that observations were illusions, because they give the "appearance" of change, when change is in fact logically impossible. When what we experience disagrees with what we can know to be true, our experience is not a reliable guide. 2500 years later, the MWIers have taken up his cause, and interestingly, in exactly the same way-- that change is impossible (if the entire closed system is included).

 

[Fra]

Yes we seem to agree in our characterization of MWI.

our experience is not a reliable guide

This is partly true, but the point is that this is the only choice we have. This IS no reliable guide. Fantasizing about it, doesn't help. We need to make decisions based upon incomplete and imperfect decision base. But so does all other systems, so it's still a fair game. At least this is my firm view.

To me the very basic quest IS a decision problem based upon incomplete information. It seems to me it should be clear why "changing perspective into one where the information is complete" is completely missing the point.

But to be consistent in the critique here, it's in place to acknowledge that there exists also platonic structures also in CI (or actually in QM itself). So it's a fair question to those (like myself) that critique MWI view why we then can accept other platonic structures. I acknowledge that point and I do take it seriously, but my take on this is not that CI solves it all (it doesn't) I rather think the problem is in QM framework itself.

/Fredrik

 

[Hurkyl]

We need to understand perception by its observable behavior. I asserted a serious of propositions earlier, things like

• Observations are outcome-valued
• If I observe a system and see outcome X and you observe the same system and see outcome Y, then X = Y. (assuming our observations are reliable)
• If I observed X and I observed Y (both observations of the same thing), then X = Y.
• If I saw X, my future actions can be affected by X

In the stronger form of MWI, it's not good enough for observers to be completely external to a quantum system. While it may be a decent approximation, we are not gods that get to see wave-functions and observable algebras and eigenvalues. Instead, we are part of the system, and we only get to interact with the system through quantum-mechanical processes.

In other words, the relevant physical properties of a system should not be things like "that particle is at position X", but instead "that observer saw that particle at position X".The challenge, now, is to consider if and how the physical properties of a quantum system can reflect the behaviors of perception.Do I have more precise details on perception? Not really -- but then I don't have precise details even from a classical viewpoint.

Doing a better job than classical mechanics is too ambitious of a goal to insist upon -- it's good enough to simply put the above on comparable footing with classical mechanics, since it means we've fully integrated into quantum mechanics what we already know and understand about perception from centuries of pre-quantum physics. That in of itself is an important advance.The central idea to get started is that we know from classical statistical mechanics that our perceptions can be reflected by probability distributions over the phase space -- by mixed states. Everett's idea was that mixtures appear naturally in relative states.

And that's the central idea behind approaches to answer the challenge -- to reinterpret observations not as the experiences of external observers as CI might do, but instead as the experiences of internal observers -- observers that are part of the quantum system -- through the appearance of mixed states (ala Everett) and a correspondence (approximately) mathematically identical to the math of statistical mechanics.

And the main thing this pushes on us is to adjust our interpretation of statistical mechanics to be of (probabilistic) parmness, rather than being of ignorance probabilities.

 

[Ken G]

In other words, the relevant physical properties of a system should not be things like "that particle is at position X", but instead "that observer saw that particle at position X".

I agree, I mentioned that earlier. Indeed, I think this is a classic duality of <observer|observation>.

Do I have more precise details on perception? Not really -- but then I don't have precise details even from a classical viewpoint.

No one does-- perception remains the core mystery. Indeed, that's why I like CI-- it starts with this recognition, so there is nothing to explain away.

And that's the central idea behind approaches to answer the challenge -- to reinterpret observations not as the experiences of external observers as CI might do, but instead as the experiences of internal observers -- observers that are part of the quantum system -- through the appearance of mixed states (ala Everett) and a correspondence (approximately) mathematically identical to the math of statistical mechanics.

Right. But CI says that observers cannot be internal to the system, because the very definition of what we mean by "a system" involves the crucial subject/object dichotomy that is the core of empirical science. The alternative is to step away from empiricism, and allow that the observer is a part of the observation, not just responsible for the observation.

And the main thing this pushes on us is to adjust our interpretation of statistical mechanics to be of (probabilistic) parmness, rather than being of ignorance probabilities.

I think you pretty much have to end up "distributing over the observers in the many worlds", rather than distributing over trials of an identical experiment. But the distribution is the same, this is not really a fundamental difference, scientifically speaking. My deepest suspicion is that there never really was any scientific difference between CI and MWI.

 

[Fra]

And that's the central idea behind approaches to answer the challenge -- to reinterpret observations not as the experiences of external observers as CI might do, but instead as the experiences of internal observers -- observers that are part of the quantum system -- through the appearance of mixed states (ala Everett) and a correspondence (approximately) mathematically identical to the math of statistical mechanics.

Right. But CI says that observers cannot be internal to the system, because the very definition of what we mean by "a system" involves the crucial subject/object dichotomy that is the core of empirical science. The alternative is to step away from empiricism, and allow that the observer is a part of the observation, not just responsible for the observation.

I agree the point of inside observers is indeed a central issue.

In CI the classical background is where the entire theory and all statistics "lives". And the classical background is never affected by the systems backreaction. Also there is no "limit" of how much information that can be stored in the classical environment. This obviously does not make sense, except in a limiting case where a large observer embraces and observes a small subsystem. This is exactly where QM is successful anyway. This also where we can say that the system under observation is closed.

When observing the entire environment or a subsystem in a large uncontrolled environment the closedness assumption fails, so we need a strategy that works for open systems. And it's exactly when we are forced to formulate a theory of sliding windows of history that the finiteness of the observer becomes important too, this leads to incomplete basis for decisions and actions and leads to decoupling of parts of the universe. Two parts interact more weakly simply because they are unable to "remember everything". I think this is even why we have problems of renormalisation with some interactions since the entire formalism is made for the "classical background" does not quite make sense when we try to consistently use it to scale it with energy.

MWI doesn't seem to address any of this. The above issues is my "problems" with CI, and they precisely have to do with lack of "inside views".

/Fredrik

 

[Fra]

[*] If I observe a system and see outcome X and you observe the same system and see outcome Y, then X = Y. (assuming our observations are reliable)

Do you by "=" informally symbolize some equivalence class? So that "X = Y" means something like "there exists a transformation depending on the observers, that transforms X into Y"?

Otherwise, then only way to comapre X and Y IMHO are to let the observers interact (here I think a little bit like Rovelli in his RQM) either with each other or with a third party than makes the comparasion, and even then it doesn't seem a priori obvious to me that the transformation that defines the eqvuivalence is known to the system making the comparasion.

/Fredrik

 

[Hurkyl]

Indeed, that's why I like CI-- it starts with this recognition, so there is nothing to explain away.

Right. But CI says that observers cannot be internal to the system, because the very definition of what we mean by "a system" involves the crucial subject/object dichotomy that is the core of empirical science.

I get the impression it was exactly the other way around.

For half it's life, QM resisted attempts to reconcile the formalism of quantum mechanics with the fact of classical observation, and there were even theoretical proofs to that effect.

The "cut" between the quantum and classical "realms" was put in place because, despite the failed attempts to reconcile them, we could at least still work with the two realms separately, and heuristically manage their interface via the Born rule (to get the classical side's statistics) and the collapse postulate (to get the quantum side's dynamics).


With no hope in sight for merging the two realms, people took the cut seriously, and thus the myriad of collapse-based interpretations.

(Okay, I guess CI can be said to "start with the cut" in the same sense that MWI can be said to "start with unitary evolution")



But half a century ago, all of the strong reasons for the necessity of the cut vanished -- Bohm resurrected de Brogile's abandoned approach, and Everett started applying relative states to the problem, and both proved to be viable interpretational approaches lying outside the scope of the old impossibility proofs didn't apply.

As an outsider looking in, it really looks more like CI is operating more through inertia than by compelling arguments, and that these alternative approaches are rapidly gaining credibility.

very definition of what we mean by "a system" involves the crucial subject/object dichotomy that is the core of empirical science. The alternative is to step away from empiricism, and allow that the observer is a part of the observation, not just responsible for the observation.

I'm not familiar with the terms "subject/object" used in this fashion. But on the following sentence, I can remark that I can reflect upon my experiences. I think think about the fact that I observed things, I can observe and think about the fact that you observe things. I can reason with these facts, and make predictions involving them.

Formally, there doesn't appear to be anything special about "I observed X".

As an aside:

a classic duality of <observer|observation>

My first thought is "two different views on the same thing" -- e.g. the duality between algebra and geometry -- and your use of bra-ket reinforces that thought.

But would I be correct that you really meant "dual" in the sense of dualism? Two fundamentally different kinds of things?

 

[Dmitry67]

No one does-- perception remains the core mystery. Indeed, that's why I like CI-- it starts with this recognition, so there is nothing to explain away.

No. CI starts from a 'measurement' which is much weaker thing - it does not involve any living and conscious beings at all.

If for you perception is axiomatic and irreductable to the behavior of big systems of QM 'particles', then you prefer a different flavor of CI (it is even not called CI):

consciousness causes the collapse

http://en.wikipedia.org/wiki/Interp...rpretation:_consciousness_causes_the_collapse

let me ask you a question, can, in pronciple, perceptor be described as QM system?

 

[Fra]

I know you asked Ken but I'll add my answer:

let me ask you a question, can, in pronciple, perceptor be described as QM system?

Yes of course, but only relative to another perceptor! Without perceptors/observers there is no place to encode/define any descriptions.

A perceptor never describes herself in terms of a QM system. The observers own state itself acts IMO like a generalization of a prior in classical probability, and there is no way for her to question herself. Just like the prior is just the "starting point" for the placement of bets and learning. If the prior is "uncertain" with respect to another observer is irrelevant to the decision process of the prior itself.

This is on par with how you can act rationally on wrong information. A corrupt prior leads to corrupted bets. But I think sometimes "corrupted bets" are exactly what explains interactions. The fact that two systems are differently informed, leads them to contradicting expectations and interactions that serves to shift the "priors". Thus by on purpose "misinforming" two parties you can predict an interaction (conflict).

But I think the big difference is that "misinformation" is really happening due to physics. There simply is not notion of "correct information", no more than there exists a measure of right and wrong that humanity can agree upon because everyone always over-weights their own benefits. That's nothing wrong, it's just how things de facto work.

/Fredrik

 

[Dmitry67]

Fra, thank you.
Here is another question (I am still interested to get an answer from Ken).

What is a difference between perception and measurement?
Is perception a measurement performed by a conscious being?
And what collapses wavefunction in CI (as Ken prefers CI) – perception of measurement?

I am trying to lead Ken into a logical trap :)

 

[Hurkyl]

Aah! I forgot the main thing I wanted to say.

We already have wonderful examples of the benefit of 'observer in the system'. The symmetries of the laws of mechanics put severe constraints on what sorts of quantities are physically meaningful... but only on the hypothesis that observers obey the laws of mechanics as well.

For example, the special relativistic notion that absolute simultaneity is not meaningful. Einstein's thought experiments -- even if we try to interpret them as involving external observers (but I don't see how to do so) -- only imply the issue is subtle. We don't get the final conclusion that it's not physically meaningful until we work out the physical laws and see the symmetry.

But if we try to say that the observers aren't part of the system and aren't described by the laws of SR, we no longer have a compelling argument -- there's no longer any reason why simultaneity must be relative for observers. If we try to recover the notion, we need a whole new set of physical postulates about the interface between observers and special relativity.

 

[Fra]

What is a difference between perception and measurement?

I think it was Ken who started to use the word perception, so I'm not sure what distinction he likes to make.

But to speak for myself, above I used perceptor as synonymous to observer.

About "measurement" someones by measurement we include the whole package of preparations etc. But I see perception more like the "detection of outcome" of an measurement.

Is perception a measurement performed by a conscious being?

Not in the way I use the term. Conscioussness has nothing to do with this. Ianimate objects also have perceptions in MY view. (Note however that my view is not the "classical CI", I'm coming from CI but have a more radical view)

And what collapses wavefunction in CI (as Ken prefers CI) – perception of measurement?

The collapse of the wavefunction is nothing but an update of the observers state (which encodes information about it's environemnt - the "system" that he observes). The key is however that there exists no "neutral description" of the observers state. It can only be described incompletely from the perspecive of other observers, or for the observer itself it is like a prior.

So the collapse, and the breaking of unitary evolution, is in my view nothing else but "updating your expectations".

The information the observer has in it's "prior" we call wavefunction (+Hamiltonian(*)), IMPLIES a change, and defines a flow of time. Because the information contains a mix of information about initial states as well as information about "motion" and momenta, together the logical implication is evolution. In my view, the SE simply is the expression for this.

This information formes a "closed system" as long as you don't get new information and is subject to "self-evolution" described by SE. When you get and information update, it would be irrational to not update the expected evolution as well.

(*) In my personal extended view, the information contained in the state vector as well as the hamiltonian are part of one uinfied structure, and the hamiltonian flow are simply defined as an entropic kind of self imposed flow. I just wanted to mention this for consistentcy, but elaborating it gets out off topic.

/Fredrik

 

[Dmitry67]

Fra, now let's wait and see how Ken G answers these questions.

 

[Hurkyl]

Do you by "=" informally symbolize some equivalence class? So that "X = Y" means something like "there exists a transformation depending on the observers, that transforms X into Y"?

No, I meant equals. X and Y outcome-valued, and the expression "X=Y" is truth-valued. The assertion is that if X and Y actually reasonably behave like their intended interpretation, then X=Y has to be (very nearly) definitely true.

OTOH, if you set up the variables X and Y in a different fashion so that their respective outcome spaces aren't intended to be related by identity but instead some other function, then I would write "f(X) = Y" for the corresponding proposition. (or some other similar thing as appropriate)

Otherwise, then only way to comapre X and Y IMHO are to let the observers interact

I agree with you in two senses, and disagree in a third.

I disagree in the sense that "X=Y" is simply a mathematical object whose behavior can be treated by the theory.

I agree in the sense that "X=Y" is only meaningful for systems that include both me and you.

I also agree in the sense that a direct empirical test of "X=Y" requires interaction (direct or indirect) between the two observers.

(here I think a little bit like Rovelli in his RQM)

Rovelli's paper gave me the final key to my understanding of interpretations. Ironically, I feel like he missed the point! (although admittedly I have not fully digested his words)

I think he made the progress of realizing that Wigner's friend using a collapsed wave-function to analyze the system after he measures it and Wigner using the uncollapsed wave-function to analyze his friend doing measurement are both just as good descriptions of 'reality'.

But he erred in retaining the correspondence between observers and wave-functions -- he failed to make the final step to detaching observers from wave-functions. There's no particular reason why Wigner's friend must switch to the collapsed wave-function to continue his calculations, and there's no particular reason why Wigner can't consider the collapses of his wave-function. Or why they couldn't calculate using a third, completely different wave-function that makes the same predictions!

A classical analog would be something like

This is Wigner's coordinate chart; it's centered on him, and he never uses any other coordinates for calculation. That is Wigner's friend's coordinate chart; it's centered on him, and he never uses any other coordinates for calculation. But happily, we can transform between them​

instead of the more sensible alternative of just saying coordinate charts are coordinate charts, and observers are observers, and they have no prior connection to one another. Anyone can use any coordinate chart they want. Coordinate charts don't have to have to be linked to observers, and so forth.

 

[Fra]

But if we try to say that the observers aren't part of the system and aren't described by the laws of SR, we no longer have a compelling argument -- there's no longer any reason why simultaneity must be relative for observers.

I think I see your point and I think it's a great example indeed. This is the connection between observer invariance or covariance and symmetries in nature that effectively encode the laws of nature.

But I think I've expressed my view on this before (here I think I am far more radical then Ken though).

There are two ways to understand this, and my hunch is that you adopt the first, I adopt the second. Both solve the problem you address but in different way. Way(1) is the realist way, way(2) is the solipsist way.

(1) Either as a constraint on the set of observers if you think the laws are timeless platonic structures.
=> the laws of nature MUST be the same as inferred by all observers.

This is in fact how it works in SR and GR. One does not talke about arbitrary observers in relativity, the theory only works for observers that belong to the same equivalence class defined by the generators of the symmetry.

The question is: what is the most general set of observers? certainly poincare transformations are hardly the most general.

(2) Or as a democracy condition if you think of the laws as emergent.
=> there is observer democracy in the sense that each observer might infer different laws, but they all contribute in negotiating the consensus and the observer invariants. In this view laws are not timeless, they are evolving. Similarly the symmetries of the laws of physics are observer dependent. As we know the poincaare symmetry which defines notoins of particles etc are completely screwed up if we consider more general observers (outside the equivalence class defined by SR itself).

/Fredrik

 

[Ken G]

The "cut" between the quantum and classical "realms" was put in place because, despite the failed attempts to reconcile them, we could at least still work with the two realms separately, and heuristically manage their interface via the Born rule (to get the classical side's statistics) and the collapse postulate (to get the quantum side's dynamics).

I think Bohr's points on this matter are clear-- there is no quantum realm, and physics is what we can say about nature. The "cut" was never a cut between two worlds, it was always the cut between the observer and the observed, it is the subject/object dichotomy that is central to all scientific models because it is the beating heart of the scientific method.

With no hope in sight for merging the two realms, people took the cut seriously, and thus the myriad of collapse-based interpretations.

Again, I find your distaste for CI to be largely around not understanding it. I wouldn't like CI either if I think it was what you think it is! CI is nothing but the interpretation that is required by empiricism, with no added bells or whistles to make it seem like a more rationalistic world. Bohm and many worlds say that reality must make sense, but CI says all you'll ever have to make sense of is your perceptions of reality, so the rest will always be angels on a pin. That's empiricism.

(Okay, I guess CI can be said to "start with the cut" in the same sense that MWI can be said to "start with unitary evolution")

Precisely.

But half a century ago, all of the strong reasons for the necessity of the cut vanished -- Bohm resurrected de Brogile's abandoned approach, and Everett started applying relative states to the problem, and both proved to be viable interpretational approaches lying outside the scope of the old impossibility proofs didn't apply.

None of those are removed empiricism. Indeed, they were both throwbacks to old ways-- Bohm is a throwback to Newtonian determinism, where initial conditions control everything that happens later, and Everett is a throwback to ancient Greek rationalism, in which the truth is the mathematics. Then when the mathematical postulates change, the truth magically follows the changes.

As an outsider looking in, it really looks more like CI is operating more through inertia than by compelling arguments, and that these alternative approaches are rapidly gaining credibility.

Empiricism isn't inertia-- is the bedrock of science. Yes, there is a lot of inertia in bedrock.

Formally, there doesn't appear to be anything special about "I observed X".

The difference is between "I observed X" and "X was observed." Science always treats those sentences as identical, hence the problem. It's not that there's anything special about me, it's just the basic issue of whether anything like me needs to be included in the description.

My first thought is "two different views on the same thing" -- e.g. the duality between algebra and geometry -- and your use of bra-ket reinforces that thought.

But would I be correct that you really meant "dual" in the sense of dualism? Two fundamentally different kinds of things?

I mean dual as in the sense that the two can be interchanged, they are two halves of the same coin, neither existing without the other.

 

[Ken G]

No. CI starts from a 'measurement' which is much weaker thing - it does not involve any living and conscious beings at all.

In empiricism, the way we know anything is because we perceive it. The way we understand measurement is how we perceive measurements. So no, measurement is not a weaker thing than perception, it is a particular type of perception, one that involves scientific constraints. I think you are confusing empiricism with the whole "tree falling in the woods" business. Empiricism does not require that someone see the tree, it merely recognizes that everything we can say about a falling tree is given meaning via our perceptions. We don't need to be there for that particular tree to recognize this fact.

If for you perception is axiomatic and irreductable to the behavior of big systems of QM 'particles', then you prefer a different flavor of CI (it is even not called CI):

consciousness causes the collapse

Empiricism views "causation" as a rationalistic notion (see Hume). Hence, the concept belongs in the toolkit of how we try to understand our perceptions. But collapse is what we perceive-- it does not require a cause. Those who imagine a physical causal link between consciousness and collapse are imagining a kind of "god's eye view" of the process. CI, and empiricism, rejects that such a perspective is physically meaningful, it is more a form of imagination.

let me ask you a question, can, in pronciple, perceptor be described as QM system?

That depends on what you mean by "be described." The empiricist translates that your question into "can we gain understanding and predictive power over the perceptor/environment relationship by treating the perceptor as a QM system? And the empiricist answer is the skeptical one: I don't know, do you have any evidence that such power is obtainable that way? But of course, that's quite a different question from what I think you mean-- is the perceptor really a QM system! The empiricist answer to that is clear: no, nor is the perceptor really anything but what it is-- a perceptor. (Here even CI may dabble in rationalism, some of the descriptions it makes tend to overstress classical thinking rather than just staying rigidly attached to the observations.)

 

[Ken G]

But I think sometimes "corrupted bets" are exactly what explains interactions. The fact that two systems are differently informed, leads them to contradicting expectations and interactions that serves to shift the "priors". Thus by on purpose "misinforming" two parties you can predict an interaction (conflict).

But I think the big difference is that "misinformation" is really happening due to physics. There simply is not notion of "correct information", no more than there exists a measure of right and wrong that humanity can agree upon because everyone always over-weights their own benefits. That's nothing wrong, it's just how things de facto work.

I see a lot of value in this view. To me, the key is the recognition that "our fingerprints" are all over the physics we do. That has in the past been viewed as some kind of a problem to be minimized, like we would minimize uncertainty in observations. But QM brings us into contact with what you get after you have minimized uncertainty, so it's time to stop seeing uncertainty (or "corrupted information") as something bad, and start seeing it as something fundamental.

 

[Ken G]

We already have wonderful examples of the benefit of 'observer in the system'. The symmetries of the laws of mechanics put severe constraints on what sorts of quantities are physically meaningful... but only on the hypothesis that observers obey the laws of mechanics as well.

Indeed, symmetries are an excellent access point to both the value, and the drawbacks, of rationalistic thinking. Symmetries are the rationalist dream-- reasoning by what has to be correct, the straight line between two points. This is also why rationalistic thinking is so powerful, and is responsible for every physical theory we have ever had. But it's also the "poison pill" of rationalism-- because symmetries were made to be broken. Consider the history of symmetries-- symmetry with time was once seen as fundamental, but then came the Big Bang model and symmetry with time was seen as only an effective symmetry. Time reversal symmetry seems quite fundamental in physics, but not in thermodynamics. Parity reversal was promising, then combinations like CT. You know the story, the history of science is a tale of one sliding rationalistic anchor right after another. This doesn't mean the rationalistic anchors are bad-- they are the very stuff of physics theories, one just needn't take them too seriously.

As for the observers obeying the laws, relativity has some nice comments on this. Relativity is a different kind of physical law, it is a meta-law-- a law that constrains laws. That right there tells you that there is something different about the role of observers in physical theory. It's not that the observer has to obey the laws, it is that truths about observers constrain the laws those observers could ever find useful. I see relativity as a kind of flashlight for looking for your keys-- it is what saves you from having to look under the lamppost, because relativity insures that the "lamp" comes with you. We look for laws like that, we need laws like that, but their purpose is to explain observations-- it is the observations that are the rock of physics, and we should expect the laws we use to understand them to constantly change as the observations broaden (but never change).

But if we try to say that the observers aren't part of the system and aren't described by the laws of SR, we no longer have a compelling argument -- there's no longer any reason why simultaneity must be relative for observers.

We cannot have that observers are part of the system in SR. Just look at the postulate: "the laws of physics shall be the same for all (inertial-- fixed in GR) observers." The postulate requires a definition of an observer be already in place, and then the meta-law constrains all the laws that could possibly be useful to an empiricist. The idea of relativity is just saying "let empiricism be an objective approach, and seek laws that can be useful within that approach", and the rest is about which brand of relativity accomplishes that (the constant c brand, for now). The observer is not "part of the system" there, or we wouldn't need relativity in the first place, we'd just have the laws of systems, and observers would automatically be covered by them, rather than appearing directly in the meta-law.

 

[Dmitry67]

But collapse is what we perceive-- it does not require a cause.

No. We don't perceive a collapse. To perceive a collapse one must at first observe a fuzzy combination of dead and alive cats, and then how it switches to a definite outcome. Nobody had ever observed it, so your claim is wrong.

For the same reason nobody had ever perceived a neutrino, instead, scientists perceive a list of numbers and retrospectively decode the behavior of the neutrino. In both cases it is the same, we perceive a shadow and we try to decode how an object look like.

 

[Hurkyl]

I think I see your point and I think it's a great example indeed. This is the connection between observer invariance or covariance and symmetries in nature that effectively encode the laws of nature.

But I think I've expressed my view on this before (here I think I am far more radical then Ken though).

There are two ways to understand this, and my hunch is that you adopt the first, I adopt the second. Both solve the problem you address but in different way. Way(1) is the realist way, way(2) is the solipsist way.

It's only a problem if you postulate that observers are external, which I don't. It's an empirical fact that I and all other observers obey Newton's laws^* (in its domain of validity^**). If there was any evidence that observers did not obey Newton's laws, scientists would be studying said observers to look for new physics.

*: Or, at least, something indistinguishable from them
**: I mean if we avoid questions like the topic of the energy levels of electrons in atoms

It's not that an observer must obey Newton's laws -- it's that we have mountains of empirical evidence for Newton's laws. The claim that observers cannot observe an absolute notion of 'at rest' is relative is merely a prediction of Newton's laws. That it is borne out in reality reflects the high level of confidence we have in Newton's laws, and that it would be violated is both very unlikely and very interesting.


Aside: symmetry is not a restriction on observers. The argument that there is no absolute notion of 'at rest' is as follows:

Consider the claim that there is some proposition P(v) of an observers velocity (v) with the property that

• P(0) = true
• P(v) = false if v is not zero

P can be thought of as the proposition that the observer is truly at rest.

We could apply a Galilean boost that adds v₀ to velocity, and we'd have a new proposition P' with the property

• P'(v₀) = true
• P'(v) = false if v is not v₀

(I've used the fact 'true', 'false', '=', 'not', and 'if ... then ...' are all invariants)

This is all still fine. We haven't contradicted anything -- P' is a different proposition than P. Nor have we refuted the claim -- this just means "at rest" corresponds to a velocity of v₀ in this representation.

But if the proposition P was actually constructed entirely out of physical laws (or any other invariants under symmetry), we must have P' = P, and so we derive a contradiction:

false = P(v₀) = P'(v₀) = P(0) = true​

 

[Hurkyl]

I think Bohr's points on this matter are clear-- there is no quantum realm, and physics is what we can say about nature.

The "quantum realm" is the part of nature that we talk about with wave-functions and unitary evolution.

Whatever ontological beliefs you may have about physics in general, you still have to recognize that collapse-based quantum physics posits that there are some things you can talk about using wave-functions and unitary evolution, some things that you can't talk about that way, and there is an interface between the two that operates via collapses and the Born rule.

The "quantum realm" is, by definition, the aspect you can talk about using wave-functions and unitary evolution.

it is the subject/object dichotomy that is central to all scientific models because it is the beating heart of the scientific method.

Your version anyways. (the phrase 'subject/object' still doesn't mean very much to me, so I'm guessing at the meaning)

I, personally, have never seen "now, make sure you have a subject/object dichotomy in your philosophical beliefs" listed in the steps of the scientific method.

Nor have I ever recall seeing anyone justify making such a dichotomy -- they just assert "do it that way".

But if you're right -- if subject/object dichotomy really is part of what "empiricism" means -- then tell me this: why should I care? Why am I not better off adopting a new philosophical position that doesn't insist on the dichotomy but is otherwise the same as empiricism?

CI is nothing but the interpretation that is required by empiricism, with no added bells or whistles to make it seem like a more rationalistic world.

The cut is a bell and/or whistle.

Indeed, they were both throwbacks to old ways--

CI is a throwback to definiteness -- to non-parmness. Well, it's not really a throwback, it's more of a carry-over from classical mechanics.

Everett is a throwback to ancient Greek rationalism, in which the truth is the mathematics.

This doesn't make sense. You keep saying it, but it doesn't make sense. The idea is that

[physics] is the general analysis of nature, conducted in order to understand how the universe behaves.

Everett wasn't trying to operate in some strange Platonic world of absolute truth. Everett was taking our general analysis of nature, and finding a way applying it to understand how the universe behaves.

The difference is between "I observed X" and "X was observed." Science always treats those sentences as identical, hence the problem.

By consistency of observation, it's not a problem -- if "O observed X" is a proposition independent of O (i.e. that "O observed X"="O' observed X" whenever both sides are well-defined), then there is no need to include O in the proposition.

 

[Hurkyl]

Consider the history of symmetries-- symmetry with time was once seen as fundamental, but then came the Big Bang model and symmetry with time was seen as only an effective symmetry.

I'm not aware of anything in BBT that suggests you can't reverse the arrow of time and get a big crunch.

Time reversal symmetry seems quite fundamental in physics, but not in thermodynamics.

Eh? It's there in thermodynamics too. It's just as ridiculously unlikely for entropy to decrease with time running forwards as it is to decrease with time running backwards.

Symmetry of the laws and symmetry of the physical configuration are very different things.

You know the story, the history of science is a tale of one sliding rationalistic anchor right after another.

... and this suddenly comes out of nowhere.

You do get the idea that our confidence in pieces of knowledge depends on empirical verification? And when new experiments are done, our confidence in some piece of knowledge can diminish? Or that our confidence in it may be surpassed by our confidence in some new piece of knowledge? And even when surpassed, our old piece of knowledge is still useful? That working with reason doesn't entail having some strange idea of absolute platonic truth and being trapped in a cycle of building a house of cards and having to completely scrap it for a new house of cards?